LLVM 3.0 Release Notes

This document contains the release notes for the LLVM Compiler
Infrastructure, release 3.0. Here we describe the status of LLVM, including
major improvements from the previous release and significant known problems.
All LLVM releases may be downloaded from
the LLVM releases web site.

Note that if you are reading this file from a Subversion checkout or the main
LLVM web page, this document applies to the next release, not the
current one. To see the release notes for a specific release, please see the
releases page.

The LLVM 3.0 distribution currently consists of code from the core LLVM
repository (which roughly includes the LLVM optimizers, code generators and
supporting tools), the Clang repository and the llvm-gcc repository. In
addition to this code, the LLVM Project includes other sub-projects that are
in development. Here we include updates on these subprojects.

Clang is an LLVM front end for the C,
C++, and Objective-C languages. Clang aims to provide a better user
experience through expressive diagnostics, a high level of conformance to
language standards, fast compilation, and low memory use. Like LLVM, Clang
provides a modular, library-based architecture that makes it suitable for
creating or integrating with other development tools. Clang is considered a
production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86
(32- and 64-bit), and for darwin/arm targets.

In the LLVM 3.0 time-frame, the Clang team has made many improvements:

Greatly improved support for building C++ applications, with greater
stability and better diagnostics.

Improved support for
the C++
2011 standard, including implementations of non-static data member
initializers, alias templates, delegating constructors, the range-based
for loop, and implicitly-generated move constructors and move assignment
operators, among others.

Implemented support for some features of the upcoming C1x standard,
including static assertions and generic selections.

Better detection of include and linking paths for system headers and
libraries, especially for Linux distributions.

DragonEgg is a
gcc plugin that replaces GCC's
optimizers and code generators with LLVM's. Currently it requires a patched
version of gcc-4.5. The plugin can target the x86-32 and x86-64 processor
families and has been used successfully on the Darwin, FreeBSD and Linux
platforms. The Ada, C, C++ and Fortran languages work well. The plugin is
capable of compiling plenty of Obj-C, Obj-C++ and Java but it is not known
whether the compiled code actually works or not!

The new LLVM compiler-rt project
is a simple library that provides an implementation of the low-level
target-specific hooks required by code generation and other runtime
components. For example, when compiling for a 32-bit target, converting a
double to a 64-bit unsigned integer is compiled into a runtime call to the
"__fixunsdfdi" function. The compiler-rt library provides highly optimized
implementations of this and other low-level routines (some are 3x faster than
the equivalent libgcc routines).

LLBrowse is an interactive viewer for LLVM modules. It can load any LLVM
module and displays its contents as an expandable tree view, facilitating an
easy way to inspect types, functions, global variables, or metadata nodes. It
is fully cross-platform, being based on the popular wxWidgets GUI
toolkit.

The VMKit project is an implementation
of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and
just-in-time compilation. As of LLVM 3.0, VMKit now supports generational
garbage collectors. The garbage collectors are provided by the MMTk
framework, and VMKit can be configured to use one of the numerous implemented
collectors of MMTk.

An exciting aspect of LLVM is that it is used as an enabling technology for
a lot of other language and tools projects. This section lists some of the
projects that have already been updated to work with LLVM 3.0.

AddressSanitizer

AddressSanitizer
uses compiler instrumentation and a specialized malloc library to find C/C++
bugs such as use-after-free and out-of-bound accesses to heap, stack, and
globals. The key feature of the tool is speed: the average slowdown
introduced by AddressSanitizer is less than 2x.

ClamAV

Clam AntiVirus is an open source (GPL)
anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail
gateways.

Since version 0.96 it
has bytecode
signatures that allow writing detections for complex malware.

It uses LLVM's JIT to speed up the execution of bytecode on X86, X86-64,
PPC32/64, falling back to its own interpreter otherwise. The git version was
updated to work with LLVM 3.0.

clReflect

clReflect is a C++
parser that uses clang/LLVM to derive a light-weight reflection database
suitable for use in game development. It comes with a very simple runtime
library for loading and querying the database, requiring no external
dependencies (including CRT), and an additional utility library for object
management and serialisation.

Cling C++ Interpreter

Cling is an interactive compiler interface
(aka C++ interpreter). It uses LLVM's JIT and clang; it currently supports
C++ and C. It has a prompt interface, runs source files, calls into shared
libraries, prints the value of expressions, even does runtime lookup of
identifiers (dynamic scopes). And it just behaves like one would expect from
an interpreter.

Crack Programming Language

Crack aims to provide
the ease of development of a scripting language with the performance of a
compiled language. The language derives concepts from C++, Java and Python,
incorporating object-oriented programming, operator overloading and strong
typing.

Eero

Eero is a fully
header-and-binary-compatible dialect of Objective-C 2.0, implemented with a
patched version of the Clang/LLVM compiler. It features a streamlined syntax,
Python-like indentation, and new operators, for improved readability and
reduced code clutter. It also has new features such as limited forms of
operator overloading and namespaces, and strict (type-and-operator-safe)
enumerations. It is inspired by languages such as Smalltalk, Python, and
Ruby.

Glasgow Haskell Compiler (GHC)

GHC is an open source, state-of-the-art programming suite for Haskell, a
standard lazy functional programming language. It includes an optimizing
static compiler generating good code for a variety of platforms, together
with an interactive system for convenient, quick development.

GHC 7.0 and onwards include an LLVM code generator, supporting LLVM 2.8 and
later. Since LLVM 2.9, GHC now includes experimental support for the ARM
platform with LLVM 3.0.

gwXscript

gwXscript is an object oriented,
aspect oriented programming language which can create both executables (ELF,
EXE) and shared libraries (DLL, SO, DYNLIB). The compiler is implemented in
its own language and translates scripts into LLVM-IR which can be optimized
and translated into native code by the LLVM framework. Source code in
gwScript contains definitions that expand the namespaces. So you can build
your project and simply 'plug out' features by removing a file. The remaining
project does not leave scars since you directly separate concerns by the
'template' feature of gwX. It is also possible to add new features to a
project by just adding files and without editing the original project. This
language is used for example to create games or content management systems
that should be extendable.

gwXscript is strongly typed and offers comfort with its native types string,
hash and array. You can easily write new libraries in gwXscript or native
code. gwXscript is type safe and users should not be able to crash your
program or execute malicious code except code that is eating CPU time.

include-what-you-use

include-what-you-use
is a tool to ensure that a file directly #includes
all .h files that provide a symbol that the file uses. It also
removes superfluous #includes from source files.

ispc: The Intel SPMD Program Compiler

ispc is a compiler for "single program,
multiple data" (SPMD) programs. It compiles a C-based SPMD programming
language to run on the SIMD units of CPUs; it often delivers 5-6x speedups on
a single core of a CPU with an 8-wide SIMD unit compared to serial code,
while still providing a clean and easy-to-understand programming model. For
an introduction to the language and its performance,
see the walkthrough of a short
example program. ispc is licensed under the BSD license.

LanguageKit and Pragmatic Smalltalk

LanguageKit is
a framework for implementing dynamic languages sharing an object model with
Objective-C. It provides static and JIT compilation using LLVM along with
its own interpreter. Pragmatic Smalltalk is a dialect of Smalltalk, built on
top of LanguageKit, that interfaces directly with Objective-C, sharing the
same object representation and message sending behaviour. These projects are
developed as part of the Étoié desktop environment.

LuaAV

LuaAV is a real-time
audiovisual scripting environment based around the Lua language and a
collection of libraries for sound, graphics, and other media protocols. LuaAV
uses LLVM and Clang to JIT compile efficient user-defined audio synthesis
routines specified in a declarative syntax.

Mono

An open source, cross-platform implementation of C# and the CLR that is
binary compatible with Microsoft.NET. Has an optional, dynamically-loaded
LLVM code generation backend in Mini, the JIT compiler.

Note that we use a Git mirror of LLVM with some patches. See:
https://github.com/mono/llvm

Portable OpenCL (pocl)

Portable OpenCL is an open source implementation of the OpenCL standard which
can be easily adapted for new targets. One of the goals of the project is
improving performance portability of OpenCL programs, avoiding the need for
target-dependent manual optimizations. A "native" target is included, which
allows running OpenCL kernels on the host (CPU).

Pure

Pure is an
algebraic/functional programming language based on term rewriting. Programs
are collections of equations which are used to evaluate expressions in a
symbolic fashion. The interpreter uses LLVM as a backend to JIT-compile Pure
programs to fast native code. Pure offers dynamic typing, eager and lazy
evaluation, lexical closures, a hygienic macro system (also based on term
rewriting), built-in list and matrix support (including list and matrix
comprehensions) and an easy-to-use interface to C and other programming
languages (including the ability to load LLVM bitcode modules, and inline C,
C++, Fortran and Faust code in Pure programs if the corresponding LLVM-enabled
compilers are installed).

Pure version 0.48 has been tested and is known to work with LLVM 3.0
(and continues to work with older LLVM releases >= 2.5).

Renderscript

Renderscript
is Android's advanced 3D graphics rendering and compute API. It provides a
portable C99-based language with extensions to facilitate common use cases
for enhancing graphics and thread level parallelism. The Renderscript
compiler frontend is based on Clang/LLVM. It emits a portable bitcode format
for the actual compiled script code, as well as reflects a Java interface for
developers to control the execution of the compiled bitcode. Executable
machine code is then generated from this bitcode by an LLVM backend on the
device. Renderscript is thus able to provide a mechanism by which Android
developers can improve performance of their applications while retaining
portability.

SAFECode

SAFECode is a memory safe C/C++
compiler built using LLVM. It takes standard, unannotated C/C++ code,
analyzes the code to ensure that memory accesses and array indexing
operations are safe, and instruments the code with run-time checks when
safety cannot be proven statically. SAFECode can be used as a debugging aid
(like Valgrind) to find and repair memory safety bugs. It can also be used
to protect code from security attacks at run-time.

The Stupid D Compiler (SDC)

The Stupid D Compiler is a
project seeking to write a self-hosting compiler for the D programming
language without using the frontend of the reference compiler (DMD).

TTA-based Co-design Environment (TCE)

TCE is a toolset for designing application-specific processors (ASP) based on
the Transport triggered architecture (TTA). The toolset provides a complete
co-design flow from C/C++ programs down to synthesizable VHDL and parallel
program binaries. Processor customization points include the register files,
function units, supported operations, and the interconnection network.

TCE uses Clang and LLVM for C/C++ language support, target independent
optimizations and also for parts of code generation. It generates new
LLVM-based code generators "on the fly" for the designed TTA processors and
loads them in to the compiler backend as runtime libraries to avoid
per-target recompilation of larger parts of the compiler chain.

Tart Programming Language

Tart is a general-purpose,
strongly typed programming language designed for application
developers. Strongly inspired by Python and C#, Tart focuses on practical
solutions for the professional software developer, while avoiding the clutter
and boilerplate of legacy languages like Java and C++. Although Tart is still
in development, the current implementation supports many features expected of
a modern programming language, such as garbage collection, powerful
bidirectional type inference, a greatly simplified syntax for template
metaprogramming, closures and function literals, reflection, operator
overloading, explicit mutability and immutability, and much more. Tart is
flexible enough to accommodate a broad range of programming styles and
philosophies, while maintaining a strong commitment to simplicity, minimalism
and elegance in design.

ThreadSanitizer

ThreadSanitizer is a
data race detector for (mostly) C and C++ code, available for Linux, Mac OS
and Windows. On different systems, we use binary instrumentation frameworks
(Valgrind and Pin) as frontends that generate the program events for the race
detection algorithm. On Linux, there's an option of using LLVM-based
compile-time instrumentation.

My current work is to use CLang's static analyzer to improve ZooLib's code
quality. I also plan to set up LLVM compiles of the demo programs and test
programs using CLang and LLVM on all the platforms that CLang, LLVM and
ZooLib all support.

LLVM IR has several new features for better support of new targets and that
expose new optimization opportunities:

One of the biggest changes is that 3.0 has a new exception handling
system. The old system used LLVM intrinsics to convey the exception handling
information to the code generator. It worked in most cases, but not
all. Inlining was especially difficult to get right. Also, the intrinsics
could be moved away from the invoke instruction, making it hard
to recover that information.

The new EH system makes exception handling a first-class member of the IR. It
adds two new instructions:

landingpad —
this instruction defines a landing pad basic block. It contains all of the
information that's needed by the code generator. It's also required to be
the first non-PHI instruction in the landing pad. In addition, a landing
pad may be jumped to only by the unwind edge of an invoke
instruction.

resume — this
instruction causes the current exception to resume traveling up the
stack. It replaces the @llvm.eh.resume intrinsic.

Converting from the old EH API to the new EH API is rather simple, because a
lot of complexity has been removed. The two intrinsics,
@llvm.eh.exception and @llvm.eh.selector have been
superceded by the landingpad instruction. Instead of generating
a call to @llvm.eh.exception and @llvm.eh.selector:

The induction variable simplification pass in 3.0 only modifies
induction variables when profitable. Sign and zero extension
elimination, linear function test replacement, loop unrolling, and
other simplifications that require induction variable analysis have
been generalized so they no longer require loops to be rewritten in a
typically suboptimal form prior to optimization. This new design
preserves more IR level information, avoids undoing earlier loop
optimizations (particularly hand-optimized loops), and no longer
strongly depends on the code generator rewriting loops a second time
in a now optimal form--an intractable problem.

The original behavior can be restored with -mllvm -enable-iv-rewrite;
however, support for this mode will be short lived. As such, bug
reports should be filed for any significant performance regressions
when moving from -mllvm -enable-iv-rewrite to the 3.0 default mode.

The LLVM Machine Code (aka MC) subsystem was created to solve a number of
problems in the realm of assembly, disassembly, object file format handling,
and a number of other related areas that CPU instruction-set level tools work
in.

The CRC32 intrinsics have been renamed. The intrinsics were previously
@llvm.x86.sse42.crc32.[8|16|32]
and @llvm.x86.sse42.crc64.[8|64]. They have been renamed to
@llvm.x86.sse42.crc32.32.[8|16|32] and
@llvm.x86.sse42.crc32.64.[8|64].

If you're already an LLVM user or developer with out-of-tree changes based on
LLVM 2.9, this section lists some "gotchas" that you may run into upgrading
from the previous release.

The LLVMC front end code was removed while separating
out language independence.

The LowerSetJmp pass wasn't used effectively by any
target and has been removed.

The old TailDup pass was not used in the standard pipeline
and was unable to update ssa form, so it has been removed.

The syntax of volatile loads and stores in IR has been changed to
"load volatile"/"store volatile". The old
syntax ("volatile load"/"volatile store")
is still accepted, but is now considered deprecated.

The old atomic intrinscs (llvm.memory.barrier and
llvm.atomic.*) are now gone. Please use the new atomic
instructions, described in the atomics guide.

Windows (32-bit)

On Win32(MinGW32 and MSVC), Windows 2000 will not be supported.
Windows XP or higher is required.

In addition, many APIs have changed in this release. Some of the major
LLVM API changes are:

The biggest and most pervasive change is that llvm::Type's are no longer
returned or accepted as 'const' values. Instead, just pass around
non-const Type's.

PHINode::reserveOperandSpace has been removed. Instead, you
must specify how many operands to reserve space for when you create the
PHINode, by passing an extra argument
into PHINode::Create.

PHINodes no longer store their incoming BasicBlocks as operands. Instead,
the list of incoming BasicBlocks is stored separately, and can be accessed
with new functions PHINode::block_begin
and PHINode::block_end.

Various functions now take an ArrayRef instead of either a
pair of pointers (or iterators) to the beginning and end of a range, or a
pointer and a length. Others now return an ArrayRef instead
of a reference to a SmallVector
or std::vector. These include:

CallInst::Create

ComputeLinearIndex (in llvm/CodeGen/Analysis.h)

ConstantArray::get

ConstantExpr::getExtractElement

ConstantExpr::getGetElementPtr

ConstantExpr::getInBoundsGetElementPtr

ConstantExpr::getIndices

ConstantExpr::getInsertElement

ConstantExpr::getWithOperands

ConstantFoldCall (in llvm/Analysis/ConstantFolding.h)

ConstantFoldInstOperands (in llvm/Analysis/ConstantFolding.h)

ConstantVector::get

DIBuilder::createComplexVariable

DIBuilder::getOrCreateArray

ExtractValueInst::Create

ExtractValueInst::getIndexedType

ExtractValueInst::getIndices

FindInsertedValue (in llvm/Analysis/ValueTracking.h)

gep_type_begin (in llvm/Support/GetElementPtrTypeIterator.h)

gep_type_end (in llvm/Support/GetElementPtrTypeIterator.h)

GetElementPtrInst::Create

GetElementPtrInst::CreateInBounds

GetElementPtrInst::getIndexedType

InsertValueInst::Create

InsertValueInst::getIndices

InvokeInst::Create

IRBuilder::CreateCall

IRBuilder::CreateExtractValue

IRBuilder::CreateGEP

IRBuilder::CreateInBoundsGEP

IRBuilder::CreateInsertValue

IRBuilder::CreateInvoke

MDNode::get

MDNode::getIfExists

MDNode::getTemporary

MDNode::getWhenValsUnresolved

SimplifyGEPInst (in llvm/Analysis/InstructionSimplify.h)

TargetData::getIndexedOffset

All forms of StringMap::getOrCreateValue have been remove
except for the one which takes a StringRef.

The LLVMBuildUnwind function from the C API was removed. The
LLVM unwind instruction has been deprecated for a long time
and isn't used by the current front-ends. So this was removed during the
exception handling rewrite.

The LLVMAddLowerSetJmpPass function from the C API was
removed because the LowerSetJmp pass was removed.

The DIBuilder interface used by front ends to encode
debugging information in the LLVM IR now expects clients to
use DIBuilder::finalize() at the end of translation unit to
complete debugging information encoding.

The way the type system works has been
rewritten: PATypeHolder and OpaqueType are gone,
and all APIs deal with Type* instead of const
Type*. If you need to create recursive structures, then create a
named structure, and use setBody() when all its elements are
built. Type merging and refining is gone too: named structures are not
merged with other structures, even if their layout is identical. (of
course anonymous structures are still uniqued by layout).

The following components of this LLVM release are either untested, known to
be broken or unreliable, or are in early development. These components
should not be relied on, and bugs should not be filed against them, but they
may be useful to some people. In particular, if you would like to work on
one of these components, please contact us on
the LLVMdev
list.

llvm-gcc is generally very stable for the C family of languages. The only
major language feature of GCC not supported by llvm-gcc is the
__builtin_apply family of builtins. However, some extensions
are only supported on some targets. For example, trampolines are only
supported on some targets (these are used when you take the address of a
nested function).

Fortran support generally works, but there are still several unresolved bugs
in Bugzilla. Please see the
tools/gfortran component for details. Note that llvm-gcc is missing major
Fortran performance work in the frontend and library that went into GCC after
4.2. If you are interested in Fortran, we recommend that you consider using
dragonegg instead.

The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being
actively maintained. If you are interested in Ada, we recommend that you
consider using dragonegg instead.

A wide variety of additional information is available on
the LLVM web page, in particular in
the documentation section. The web page
also contains versions of the API documentation which is up-to-date with the
Subversion version of the source code. You can access versions of these
documents specific to this release by going into the "llvm/doc/"
directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact
us via the mailing lists.